Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Vehicles can be configured to operate in an autonomous mode in which the vehicle navigates through an environment with little or no input from a driver. Such autonomous vehicles can include one or more sensors that are configured to detect information about the environment in which the vehicle operates. The vehicle and its associated computer-implemented controller use the detected information to navigate through the environment. For example, if the sensor(s) detect that the vehicle is approaching an obstacle, as determined by the computer-implemented controller, the controller adjusts the vehicle's directional controls to cause the vehicle to navigate around the obstacle.
A variety of vehicle control systems rely on information output from wheel speed sensors to provide automatic adjustments in vehicle handling and control. For example, anti-lock braking systems are electronically controlled to modify braking forces applied to each wheel in a vehicle to increase vehicle control during braking. At least in some instances, anti-lock braking systems decrease stopping distances and also allow for greater vehicle maneuverability even during braking. For example, an anti-lock brake system can be configured to detect instances where wheels are locked, indicating loss of traction, and automatically decrease braking forces applied to locked wheels.
Similarly, stability control systems, and adaptive power distribution systems can automatically adjust torque applied to different wheels to account for differential traction of some wheels due to wheel slip on loose surfaces, or to provide assistance during hard turning by selectively braking wheels based on inferred traction of each wheel indicated by the speed sensor. Some systems can also decrease rollover risks by distributing braking forces across the wheels based on the center of mass of the vehicle and steering information from the steering wheel.
Wheel speed information is provided by encoders on each wheel. A single channel encoder outputs square wave pulses with a fixed number of pulses per complete rotation of each monitored wheel. In some instances, the single channel encoders can operate by Hall Effect or magnetoresistive sensors detecting regularly spaced magnetic and/or metallic features on the rotating portion of the wheel. The frequency of the output pulses thus provides an indication of the rotation rate of the wheel. However, such square wave pulse trains do not provide an indication of the direction of rotation of the wheels. A single channel encoder does not distinguish between a feature passing the sensor in one direction and passing the sensor in the opposite direction, and so the pulse output from a wheel rotating in one direction is generally indistinguishable from the pulse output with the wheel rotating in the opposite direction.
Dual channel encoders can distinguish direction of rotation by including a second sensor that detects a second set of features on the rotating wheel. The second set of features is axially offset from the first such that the order of detection of the two sets of features is different when the wheel rotates forward than when it rotates backward. Thus, dual channel encoders provide both speed and direction information, but require additional hardware and associated cost.